1. Field of the Invention
The present invention relates to an apparatus for and a method of capturing a radiation image by emitting radiation from a radiation source and applying the emitted radiation to a subject.
2. Description of the Related Art
In the medical field, for example, it has been customary to apply radiation to a subject and to detect the amount of radiation that has passed through the subject with a radiation detector, or to guide the radiation that has passed through the subject directly to an X-ray film or the like, thereby forming a radiation image of the subject for diagnostic purposes.
For obtaining a radiation image suitable for image interpretation and diagnosis, a tube voltage, a tube current, and a radiation application time are established as appropriate exposure conditions, in view of the body region to be imaged and other characteristics of the radiation source.
According to the conventional X-ray image capturing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2004-209152, parameters such as thickness data produced when the thickness of a subject is measured, the tube voltage of the radiation source, the distance from the radiation source to the radiation detector, the sensitivity of the radiation source, and the absorption rate at which the subject absorbs radiation (radiation attenuation coefficient) are established. Then, a tube current and a radiation application time are calculated using the established parameters.
Ordinary radiation image capturing apparatus are associated with a device (exposure control device) for detecting a radiation dosage that has passed through a subject, or a radiation dosage that is applied in the vicinity of the subject, when a radiation image of the subject is captured under the exposure conditions referred to above, whereupon application of the radiation is automatically stopped when the detected radiation dosage reaches a required radiation dosage that has been determined from the exposure conditions (see Japanese Utility Model Registration No. 2607256). The exposure control device allows the subject to be irradiated with an adequate amount of radiation for capturing a suitable radiation image of the subject, while also preventing the subject from being irradiated excessively.
When a radiation dosage applied to a subject is controlled by such an exposure control device, if an implant is embedded in the breast of a subject, for example, then the dose of radiation that passes through the subject becomes smaller than normal. Therefore, even if the radiation dose applied to the subject is excessively high, the radiation dose may possibly be detected as a radiation dosage that is lower than the required radiation dosage. Furthermore, an excessive radiation dosage cannot be detected in the event of a failure of the exposure control device, including failure of the radiation detector therein.